1. Field of the Invention
The present invention relates to methods of and apparatus for incrementally advancing a carriage when conducting a raster scan. More particularly, the instant invention relates to methods of and apparatus for incrementally advancing a carriage which is used, for example, to provide relative movement between a recording device and receiving medium in a raster scan system.
2. State of the Prior Art
Laser printers include means for moving a scan head after each line is recorded on a recording medium. In a known apparatus, a lead screw is used to advance a carriage supporting the scan head in the "slow scan" direction. A problem in using lead screws is that artifacts may occur as a result of very minute variations in lead screw pitch. These artifacts can appear as repetitive patterns, or "banding" in the image produced on the recording medium. The eye is extremely sensitive to periodic variations in density of an image, especially in areas which are nominally of uniform tone. In order to achieve band-free images in scanner apparatus which utilize lead screws, extremely high precision lead screws are necessary wherein the lead screws are ground and lapped to perfection. In addition, the lead screws themselves are driven by high precision motors. Accordingly, minimizing banding is too expensive for many applications.
Other techniques, such as those disclosed by U.S. Pat. Nos. 4,608,578 and 4,505,578 disclose utilizing a hydraulic cylinder to control the movement of a carriage on which a photosensitive medium or scanning mechanism is placed. In each of these patents, a braked, gravity transport is provided for moving a carriage. The carriage is propelled by a falling mass which works against a piston that is supported in a cylinder containing hydraulic fluid. A valve limits the flow of hydraulic fluid out of the cylinder so that the fall of the mass, and hence the carriage, is braked to a uniform velocity. The velocity of the carriage is controlled by controlling the rate of fluid flow through the valve. A difficulty with this device for many applications is that it is very difficult to obtain a uniform velocity throughout the full extent of carriage movement. The carriage velocity, and therefore its precise position at any specified time, depends upon a delicate balance between the force of gravity, the hydraulic braking force, and the force of friction between the moving parts, such as the carriage on its rails, or the hydraulic seals on the cylinder bore, etc. Since friction is notoriously variable, the carriage velocity can be expected to be variable. Further problems with this apparatus are that the apparatus is not compact and the falling mass must necessarily be oriented vertically.
U.S. Pat. No. 4,433,346 discloses a scanning mechanism utilizing a linear motor; however, the linear motor is not disclosed as being configured in a manner to eliminate banding. U.S. Pat. No. 4,562,349 utilizes hydraulic cylinders to advance a support arm to which a scanning head is attached; however, again, the arrangement is not configured to minimize banding.
U.S. Pat. No. 3,344,526 is directed to an apparatus for minimizing periodic errors when ruling diffraction gratings, wherein lines are engraved on a reflector blank. The blank is advanced by a hydraulic ram as discrete volumes of hydraulic fluid are accumulated in a hydraulic cylinder containing the ram to extend the ram therefrom in a series of discrete steps. This approach has yet to be used in or considered for raster printing, rather the aforementioned approaches with their attendant disadvantages have been employed or considered.